Circadian rhythms and environmental lighting regulate a number of endocrine and behavioral functions. Dispersed chick pineal cells remain rhythmic and responsive to light in culture. Light has two effects on the melatonin rhythm: acute suppression and entertainment (phase shifts) of the underlying clock. The key enzyme in melatonin synthesis, serotonin N-acetyltransferase was partially purified, separated and distinguished from a related, drug-metabolizing, enzyme. Caffeine, at concentrations that release calcium from intracellular stores, induced light-like phase shifts in the melatonin rhythm while increasing melatonin synthesis. Thapsigargin and EGTA each blocked the entraining effects of caffeine and of light. These results point toward a role for intracellular calcium fluxes in entertainment of the circadian pacemaker.